Plural-beam growing-wave tube



March 3, 1953 w. J. DODDS 2,630,547

PLURAL BEAM GROWING-WAVE TUBE Filed July 27, 1949 2 SHEETSSHEET l 16 P15 i l l l bi l i l l INVENTOR I l llesleyi Dad'ds March 3, 1953 w. J.DODDS PLURAL BEAM GROWING-WAVE TUBE 2 SHEETSSHEET 2 Filed July 27, 1949S atam l m m a vm mu D m Q V 2 Q? Q. m F m a am 3 1 N m 55 mm Q 8 3w w Yi w I\ 1 H R m. .m wk? @m mm m Sui 9 N mu W A? 3.. 3 mm w. Q S Q, x 1% Km l/ I 1/111 1/ I 1 44444444 l l 1/ z r I W NW kbnxkbQ .Vm. M Q Q 9 @qmm Q 3 mm mm 3 & m w mm k\\\\ NM NM Q 00 WW mm w Hmv H E2 mm 3 3 W N mmQlm m mm n m ww x 3 w @j/ z 1/ mm 1 1/ z 11 z zv z z z r z z v PatentedMar. 3, 1953 PLURAL-BEAM GROWING-WAVE TUBE Wellesley J. Dodds, Cranbury,N. J., assignor to Radio Corporation of Americana corporation ofDelaware Application July 27, 1949, Serial No. 106,986

12 Claims.

This invention relates to improvements in electron discharge devices forgenerating and amplifying energy in the ultra and super high frequencyregions. More particularly, it relates to improvements in traveling wavetubes of the type variously known as plural beam tubes, mixed-beamgrowing-wave tubes, and electron wave tubes.

This type of tube represents a significant advance in the traveling wavetube art because of its remarkable property of achieving energyamplification without the use of field-supporting or wave-guidingstructures in the amplifying region of the tube, i. e., in the driftspace. In it provision is made whereby the function of carrying highfrequency waves slowly along a drift space in close proximity to astream of moving electrons (herein referred to as a first group ofelectrons) is effectively performed by other moving electrons (hereinreferred to as a second group of electrons) instead of by a passivecircuit element such as a delay line or helix. Under the influence ofinput signals applied to a control element at least one of the groups ofelectrons is made to form itself into waves which grow as they movethrough the drift space, the increase in wave energy being attained atthe cost of a lessening of an initial difierential which is caused toexist between the kinetic energies of the two groups of electrons and bya phenomenon which has been described as a mechanism of elasticcollisions between fast and slow electrons. In actual practice bothgroups of electrons are caused to form themselves into waves, ratherthan to shoot one of them through the drift space as a D. C. stream, asthis seems to cause the above mentioned mechanism of collisions tooperate to greatest effect, i. e., to afford the greatest gain.

In the prior art, as typified by the article on page 4 of the January1949 issue of the Proceedings of the I. R. E., volume 31, No. 1, thestream of intermixed "groups of electrons of different speeds has beenformed in either of two ways. One way has been to produce a singleinhomogeneous beam in which, due to space charge effects, the outerelectrons (one group) travel with greater velocity than the innerelectrons (another group). The other way has been to produce a pluralityof beams, each of which may be homogeneous, by arrangements includingthe use of separate cathodes which are so positioned, either behind oneanother or closely juxtaposed side-by-side, that the beams willintermix. The practice of accepting the limitation of necessarilyproducing the intermixed stream in one of these ways has resulted in anumber of disadvantageous structural arrangements for these tubes. Forexample, in every prior art arrangement for a tube of this kind thegroups of electrons are intermixed in a region which they enter soonafter they leave their common or respective cathodes (as the case maybe) and thereafter it is not until they have traveled for some distanceas an intermixed stream that they pass through a control element such asa resonant input cavity or an input helix, where they can be influencedby the input signals. This results in more growing-wave amplificationfor noise than for useful signals, since noise waves of intermixedelectrons travel further in the drift space than do signal waves. Ofcourse, if it were possible for the respective intermixed beams to haveabsolutely constant currents and velocities, then there would be nonoise amplification at all since they only interact upon each other ifat least one of them includes wave components. But, as is known, this isnot possible since the electrons from each cathode will be emitted fromit with the usual random irregularities of initial direction andvelocity, which are responsible for shot noise. As a result noise waveswill be present in each admixed beam and these waves will already beinteracting to produce gain in the portion of the drift space betweenthe place where intermixing starts and that where the input signal isimpressed on the intermixed stream. For this reason in prior artmixed-beam traveling-wave tubes the signal to noise ratios have beenlower than those which, as will be seen, are now attainable according tothe present invention.

It can be mathematically demonstrated that the amount by which thesignal-to-noise ratio of prior art electron wave tubes suffers is quitesubstantial. This is due to the fact that the gain of this type of tubeis a function of the length of drift space in which interaction for aparticular signal takes place. This function is set forth in extenso inthe article Analysis of a simple model of a two-beam growing-wave tube,by L. S. Nergaard, RCA Review, December 1948. Because of the infinitefrequency spectrum of noise currents very effective interaction occursbetween two sets of shot noise electron waves. The distance over whichpre-amplification of noise may thus occur in prior art tubes isfrequently of the order of 10 per cent of the entire drift space. (SeeFigs. 10 and 12 of the article which is identified above as appearing inthe January 1949 issue of the Proceedings of the I. R. E.) On this basisthe shot noise which originates at the cathodes of a tube which affordsa gain of 60 decibels would be substantially 6 decibels greater onreaching the signal input point than when the beams first intermingled,thus worsening the tube noise factor by 6 decibels.

Moreover, particular kinds of prior art tubes have additionaldisadvantages. In one such kind two spiral cathodes are used, one beingpositioned behind the other, and intermixing is .attained in that thegroup of electrons produced from the first cathode passes through the.open structure of the second cathode. This has the disadvantage thatthe second cathode draws some of the current from the first one. Inaddition, any adjustment of the polarizing potential applied to onecathode will effectively alter the adjustment of that applied to theother. A final difficulty with this kind of tube is that partition noisewill be generated as a result of partial interception of electroncurrent from the first cathode by .the structure of the second :one.

Accordingly, it is an object of the present invention to deviseimprovements in mixed-beam growing-wave tubes whereby the groups ofelectrons which are to interact with each other in the drift space ofthe tube will not be intermixed until after the input signals havebeenimpressed on each of them.

It is a further object of the present invention to devise improvementsin mixed-beam growingwave tubes for keeping down the generation ofpartition noise and for permitting adjustments of the velocity andcurrent of any one electron beam without thereby unduly influencing thevelocity and current of any other beam, the improvements being attainedby novel arrangements in which none of the electron beams to beintermixed passes through any structural element of the electron gunproducing any of the others.

It is a further object of the present invention to devise improvedmixed-beam growing-wave tubes in which the drift path for each of thebeams includes a first and individual portion in which the beam movesalone without intermingling with any other anda final and common portionin which all of the beams intermingle, move together, and interactwitheach other, and in which a means-is provided for impressing theinjut signal on all of the beams in the individual first portions oftheir drift paths in order to attain signal wave interaction andamplification over all of the common final portions of their paths.

It is a further object of this invention to devise mixed-beamgrowing-wave tubes as set forth in the preceding paragraph in whichsaidlast mentioned means comprises a separate means for modulating eachof the beams with an inputsignal whereby, in addition to attaining goodsignalto-noise ratio, it will be possible to employ aseparate means inthe external circuit which is used to impress the input signal on eachbeam ordetermining the phase and magnitude of the electron waves to beformed on it.

It is afurther object of this invention to devise improvements formixed-beam growing-wave tubes in which individual electron guns areprovided for respectively producing the several beams, each gunincluding means for adjusting the current and velocity of the beam whichit produces and being spaced from all of theother .electrons.

4 guns so that said means may be ajusted without affecting the beamproduced by any of them.

Other objects, features and advantages of this invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments of this invention and from thedrawing in which:

Figs. 1 and 2 illustrate schematically two representative prior artarrangements for producing an intermixed electron beam in a travelingwave tube; v

Fig. .3 represents the trajectories of two electrons traveling at auniform rate of speed in a :non-uniform magnetic field, which variesaccording to a simple law, and illustrates a principle which underliesthe present invention;

Fig. dis a sectional view of one embodiment of the present invention;and

.Fig. 5 is a sectional view of an alternate embodiment of the presentinvention.

Fig. 1 represents a prior art arrangement for producing a :beam :of twointermixed groups of Two fiat, spiral cathodes it and jil are arrangedalong an axis l2 of .a-;.growing-,waye tube, not fully shown in Fig. 1,in suchawvay that electrons emitted by the cathode H] can penc tratebetween the .turns of the spiralof the oathode H in order to reach thedrift space I13 ,(of which only the portion near to the-electron-sourceis shown). In thearrangementof :Fig. 31 a single accelerating electrodeM is employed. {Eheemitted electrons are caused to ,follow'pathsparallel to the axis l2 by means of ,a strongaxial'magnetic fieldproduced by a sourceof magnetic flux which is not shown.

A source 55 of an accelerating potential of n volts is applied betweenthe-cathode all .and the electrode i i to cause the electrons from thecathode ii to attain a velocity of n volts before they enter the driftspace 113. ,Anothersource, it, which provides a potential .of m'voltsis.connected in series with the source it :between the cathode Ill andthe' electrode M so that the :electrons from the cathode iii enterthedrift space at the velocity m+n voits. .Accorldinglytwo groups ofelectrons move together through the adrift space each group with adifferent velocity and each group having a predetermined:cu-rrentdensity controllable in .any suitable .linown Qmanner. Thisarrangement of isemrtranspanent rcath odes has the advantage that it.afiords a very thorough mixing of the two electron streams. An inputcavity ii is placed :so that its grids l3 and is lie athwart thepath-10f the electrons, i. e., are transverse to the {drift space. Whenproperly excited the input cavity will ract ina known manner to velocitymodulate the twobeams of electrons which move between its grids it, It,thus forming two sets of electron waves. While it is true that over acertain-length of drift space each electron wave mayigrow in .ma nitudeas a result of blanching-far greatergrowth is possible as due tointeraction between the two Sets of Waves.

It should .be noted that in .this arrangement the two electron beams areintermixed for the entire distance from the cathode ii .to the grid 68before any input signal acts upon them. Because of this, shot noisewaves carried .by these beams will undergo acertain amount ofamplification before the input signal is impressed on the beams. Thismay preclude the use of this tube in cases where the input signal -,-isexpected to be of the order of a few micro volts ,-e. g., where it is tobe used as the R. F. input stageforcertain kinds of super high frequencyreceivers, and this is especially unfortunate because of its potentiallyvery great sensitivity. Nor would the situation be corrected simply byputting another input cavity between the two cathodes I0 and H since thewaves which the added cavity could be made to impress on the electronsfrom cathode I0 would not receive any substantial amplification byinteraction with the beam of electrons from cathode ll until theyreached the cavity H, i. e. until the point where signal waves were alsoformed on this beam. This is true because the pluralbeam growing-wavephenomenon results primarily from the interaction of a plurality" ofwaves. In fact, the situation would be worsened since the partitionnoise, which would be generated by partial interception of the electronsfrom the cathode I!) by the added input cavity, would produce noisewaves which, due to their infinite spectrum, would be amplified in theregion between the cathode II and the grid I8.

The prior art arrangement of Fig. 2 has much in common with that of Fig.1 except that the cathodes 20 and 2 l, instead of being directly heatedspirals placed one behind the other along the tube axis, consist ofindirectly heated structures which have concentric emissive surfaceslying in approximately the same plane and are insulatingly supportedwith respect to each other.

In this arrangement the problem Of partition noise is lessenedconsiderably. However, as in Fig. 1 a substantial amount of noiseamplification will take place before the input signal is impressed onthe intermixed beams.

In general, according to the present invention: the electron beams whichare to be intermixed are produced by separate electron guns which arenot juxtaposed but rather are spaced apart in regions where they willnot influence each other and in an arrangement which permits each beam Ito be individually controlled as to the density and mean velocity of itselectrons and to be individually influenced by the input signal; thetube envelope encloses space providing a single common drift path inwhich the beams may move together after they have been intermixed, and anumber of individual drift paths between the respective guns and aregion where the beams converge into the common drift path; and anon-uniform magnetic field is used for causing the electron beams thusproduced at widely separated points to converge to the common drift pathand thereafter to move in substantially the same direction along it.

It can be shown that charged particles, in particular electrons, can becaused to follow trajectories such as those shown in Fig. 3, if theyhave a constant velocity and move in a magnetic field perpendicular tothe plane of Fig. 3 which is constant in every ZY plane but which variesin a predetermined manner in every Z--X plane. This is shown, forexample, in the article by Coggshall and Muskat which appears inPhysical Review, volume 66, Nos. '7 and 8, pp. 187-198, October 1 and15, 1944. In this article an analysis is made of the behavior of chargedparticles which move at constant velocities into magnetic fields whichvary in certain fairly simple ways which are amenable to analysis interms of Cartesian or polar co-ordinates.

The present invention makes use of the fact that two electrons can bemade to follow respective paths such as those shown in Fig. 3 and inparticular the portions of those paths between the lines A--A and BB.Accordingly for each '6 of the embodiments of growing wave tubes, 1. e.for each of the embodiments respectively shown in Figs. 4 and 5, theevacuated envelope 30, which serves as an electrostatic shielding means,is formed to provide one common drift space into which the paths of anumber of beams may converge, and along which they may then followapproximately the same course, and a number of individual drift spaceseach affording a path of travel for one of the beams from its point oforigin to its point of convergence into the common drift space. To thisend envelope 30, which is shown herein by way of example, comprises amain tubular enclosure 3|, which surrounds the common drift space, and anumber of branch tubular enclosures 32 (such as two) each of whichsurrounds one of the individual drift spaces. Each branch tubularenclosure 32 has one end smoothly joined to the input end of the maintubular enclosure 3! and all of the branch enclosures extend from saidinput end in divergent curved directions far enough so that their otherends, 33 are in regions offset from the axis of the main tubularenclosure and are substantially spaced from one another.

Each of the branch enclosures 32 encloses in the region near to its end33 an electron gun comprising a heater 34, an indirectly heated cathode3, a control and/or accelerating grid 8, and a final acceleratingelectrode 35. Each electron gun 34 should be designed, as is possible inaccordance with principles which are well known in the art, to produce abeam of electrons which emerges from the final accelerating electrode ata predetermined drift velocity and in a predetermined direction. Due tothe electrostatic shielding effect Of the various parts of the envelope3!], the individual electron guns are shielded from each other, and allof the drift spaces within the envelope 30, including those in itsbranch tubular enclosures 32 and that in its main tubular enclosure 3|,are field-free so that once any group of electrons have left their gunit will tend to move with a constant average velocity. In other words,no added electrostatic acceleration is employed after the differentbeams of electrons leave their respective guns. For this reason in theopera.- tion of a tube of this kind the final accelerating electrode ofeach gun will usually be maintained at the same potential as theenvelope 3!] and the other gun elements will be at lower (less positiveor more negative) potentials.

Preferably the final accelerating electrode 35 should contain an orificefor defining the crosssectional shape of the beam. In the drawing eachof these electrodes is shown as a thin conductive partition mountedtransversely across the inside of its respective branch enclosure 32between the end portion thereof which encloses all of the other gunelements and the remaining portion thereof which encloses a curvedindividual drift space to be followed by the beam after it emerges fromthe beam-defining orifice. To form a suitable beam defining orifice onemay cut through the partition a thin slit the axis of which isperpendicular to the plane of the drawing so that the beam will assumethe form of a thin sheet of electrons. Accordingly the line 36 by whicheach electron beam is represented in the drawing may be considered asthe edge of the ribbon-shaped beam. The axis of each electron gun shouldbe directed so that, in the absence of any magnetic influence, its beam,on emerging from the electrode 35, will tend to travel in the directionof a tangent to a particular point on a particular 7 murve,fi.'e.,-to-.a point in the center of 'thelb'eamrdefining "orifice .on acurve which may be deitermined-aaccording 5170 theiteachings. of.c'oggshall .Ifzthegun isso directed, then, under the influenceofanappropriate magnetic field, each beam will begin to .follow theintended curved path of "travel immediately upon its emergence from thebeam-defining orifice and its entry into the field- :free space "withinthe envelope 30.

If all of the .beams were to travel at exactly the same velocity in theappropriate magnetic field and if that velocity were the correct oneifor producing .tra'jectoriesas in. Fig. 3, then each vof Ethe:beams'would approach very close to the tube axis (in the case ofsheet-shaped beams, :very close .toasp-lane which includes the tube axislandiszperpendicularto the plane of the drawing) and they would thustravel side by side inclose proximity to one another over a considerableportioniof the common drift space. Such proximity in combination withthe intermingling of fringing electrons wouldat least partiallysatisfythe ;requi-rement as to intermingling which must be imetif thebeams are to interact to cause travelingewave growth. However, anotherrequirementemust also be .met, namely, the beams must adjust thevelocities of the beams to values slight- 1y different from'the correctones for producing the exact trajectories shown in Fig. 3 so assimultaneously'to improve intermixing, by causing the trajectoriestocoincide or cross over one another in their central regions, and toobtain the :velocity differential which is needed for stronginteraction.

In practice the beam velocities and currents .ordinarily will becontrolled by adjusting the magnitudes .of negative potentials appliedrespectively to the different elements of the differ- :ent :guns whilethe final accelerating elements .35, the envelope 3'0, and the positiveterminal of the :source of direct potential are 'all at groundpotential. Though certain theoretical consider- :ations indicate thateach of the electrodes should alwaysibe exactly at the potential of theenvelope '30, in practice it will often'prove to be (advantageous toestablish a verysmall (adjustab'le') potential difference between them,such as :a difference .of one or two volts. For this reasonand in orderto :provide an add-ed control :parametenit maybe advantageous toinsulate leach final accelerating electrode 35 from the inner walls ofits branch tubular enclosure 32 and to provide a llead which extendsfrom it through a vacuum seal to the outside of the envelopeBG toprovide a terminal suitable for connection to an external circuit.

Atthe end 33 of each branch tubular enclosure 32 there is an insulatingvacuum seal 3! through which leads maybe fused for connecting theelectron gun elements to external circuits including sourcesofpotential.

A-means is provided for impressing 'the input signal on each beam whileit is still moving in the branch tubular enclosure 32 in which itoriginated. In theembodimentof Fig. leach of these means consists of1anlin'put resonant cavity 338 which isconnected between itwouportionsrof arespectivebranch tubularenc'losure byrva'cuumtight junctures whichextend unbroken all around thezperipheries of thegrids 3910f the cavity,inla manner known in the art of velocity- 'modulated inductive-outputtubes, so that the body of the-cavity=constitutes a part of theevacuated envelope of the tube.

Since the beam produced by the embodiment of Fig. 4 is sheet-shaped, themain tubular en- *closure 3l and each of the branch "tubular enclosures32 should have a. cross-sectiomsuch as an .oval or rectangle, which hasonetransverse axis which is muoh longer than :the other (the longerofthe axes being disposed in a direction which is perpendicular out of theplane of the drawing) and eac'h of the resonant cavities :38 should beappropriately formed to fit around sucha branch enclosure. .In other.words, an outside view of-one of the cavities taken from-apos'ition'near one of the ends 33 should reveal it to' have an oval orrectangular shape.

"Each of the inputcavities 38 is provided with an input coaxial line 40the inner-conductor of which may be inductively coupled to a portion ofthe space inside of the cavity, each coaxial line All has anelectromagnetic-energy-permeable vacuum-tight bushing H sealed betweenits inner and outerconduc'tors. To illustrate one way of using this typeof tube the input lines are shown connected to an appropriate commonexternal inputcircuit. The external input circuit, shown herein only byway of example, comprises a lead-in coaxial'line i2,a T connector 43,-bywhich energy supplied over the lead-in line 42 will be delivered to twobranch lead-in lines 44, and two phase-and/or-magnitude controllingdevices 45, each of which has an input connected to one of the branchlead-in lines as and an-output connected to one of the input-coaxiallines t0. "This arrangement makes it possible individually tocontrolwith what magnitudes and/or phases the "input signal will beapplied to the different beams.

It is apparent that in a growing-wave tube according 'to this inventionsignal waves are impressed -on all of the beams before they are itermixed as a result of which the signal Waves 'will receive as muchamplification in the common drift spaceas will noise waves.

the output end of the main tubular enclosure 31 there is located anoutput resonant cavity 45 which, in a known manner, will eX- tractenergy from the waves of intermixed electrons'which pass between itsgrids ll. TheOllllput cavity is provided with an output coaxial line 38which has its inner conductor inductively coupled to the space inside ofthe cavity; has an electromagnetic-energy-permeable vacuumtig'ht bushing39 sealed between its inner and outer conductors; and is provided with aconnector fitting by which a load maybe connected to the tube. Beyondthe output cavity 16, in the direction of electron travel, there is acollector 51 which is insulatingly adjoined to the endof the 'maintubular enclosure 3! by a vacuum-tight seal 52.

The fullest utilization of the interaction between. the intermixedbeams, and therefore the maximum gain, will be attained by making thecommon drift space so long that the beams will start to be divergent inthe region between the output cavity 46 and the collector "5|. Thedivergence will not be objectionable even if it is so large that thebeams strike the inner walls of the collector since, as a matter offact, it is the function intended for this element that it intercept allof the electrons which have passed through the drift space so as tostart them on their return (through the power supply) to the cathodes ofthe different electron guns. A is usual practice, the collector will beoperated at a potential somewhat higher than that of the electrons, i.e., somewhat more positive than the tube envelope 3!].

In the operation of this tube it is necessary for each beam of electronsto be magnetically influenced in order to cause it to follow a curvedpath until it reaches the input end of the common drift space andthereafter to follow analmost rectilinear path along the common driftspace. To this end the entire envelope of this tube is fabricated of anon-magnetic material and, during its operation it is immersed into amagnetic field of the kind described by Coggshall and Muskat ascausingcharged particles to follow the trajectories shown in Fig. 3. Anonmagnetic material is required, for example, copper, so that asteady-state magnetic flux can readily penetrate the envelope totraverse the drift spaces which it surrounds and so that it can do thiswithout having the magnetic field configuration materially distorted.

An appropriate source of magnetic flux and an appropriate magneticcircuit may be designed according to any of a number of wellknownprinciples by the use of electromagnets, as represented at 6B and SI, orpermanent magnets, or combinations of both. In the drawin I have broadlyindicated the directions of ma netic flux lines of the field into whichthe tube is immersed during its operation, i. e., by a symbol near tothe reference character, MFi (magnetic flux in), which indicates that onthe upper side of the envelope, as shown in Figs. 4 and 5, the fluxlines are normal to and downward into the plane of the drawing, and by asymbol near to the reference character, MFo (magnetic flux out), whichindicate that on the lower side of the envelope the flux lines arenormal to and upward out of the plane of the drawing.

The magnetic field should be large enough so that all portions of thetube structure, including its branch enclosures 32, all the way back totheir respective electron guns, can be fully immersed in it. In fact,its fringe portions should be located well beyond the ends of the tubeso that exactly the desired magnetic flux distribution can be attainedfor all of the magnetic field into which the tube is immersed. The fluxdensity of the portion of the field which passes through the tubeenvelope should be constant in each plane which is perpendicular to theplane of the drawing and extends in the Z-Y direction and it should beunequal in adiacent ones of said planes increasing progressively inmoving in the ZX direction. Since the fiux lines are upward on one sideof the envelope and downward on the other there will be a trough of zeroflux in a plane, herein designated the axial plane of the tube, which isvertical to the plane of the drawing and extends along the centrallongitudinal axis of the tube.

As represented in Fig. 4, an appropriate magnetic flux source andcircuit may comprise one long, fiat core piece 62 or 63 placed on eachside of the tube to extend somewhat beyond both ends thereof and to liein a plane parallel to 10' the axial plane of the tube. Obviously to bethus positioned each core piece must be positioned at least as far awayfrom said plane as the outermost protrusion (such as an electron gunlead) from the end 33 of the branch enclosure 32 which is on the sameside of the tube axis. Windings, represented respectively at 64 and 65,are wound longitudinally around each core piece with each turn thereofsubstantially parallel to the plane of the drawing so that directcurrent excitation will induce magnetic lines of force which areperpendicular to the plane of the drawing. In order to make it possibleto adjust the flux gradient between th center of the core piece and theabove-mentioned trough of zero flux, any suitable mechanical means maybe provided whereby the electro-magnet may be moved, as a whole, towardor away from the tube (while at the same time maintaining its abovementioned parallelism with the axial plan of the tube), and any suitableconventional means may be provided for varying the D. C. current throughthe magnet. In addition, if necessary the magnetic circuit may be madefurther adjustable by a trial-and-error procedure including insertingone at a time or in diiferent combinations longitudinal laminations, notshown, which have different characteristics, such as differentreluctance values and/or thicknesses, at different positions between thecore pieces and parallel thereto to attain the desired fieldconfiguration.

The embodiment of Fig. 5 is in all respects similar to that of Fig. 4with the exception that its two signal input devices and its one signaloutput device consist respectively of helices 53-, 53 and 54, ratherthan of resonant cavities. As shown in Fig. 5, each of these helices maybe connected in a convetnional manner, i. e., one of its ends may beconnected to the inner conductor of the input coaxial line which feedsit and the other may be connected to its outer conductor via a portionof the conductive inner surface of the envelope 30. As is known, eachhelix must be sufficiently spacer from the adjacent conductive innerwall of the envelope to avoid inductively short-circuiting any signalwhich it is carrying.

While the view of each of the helices which is afforded by Fig. 5 maycause its turns to seem circular, actually they are nearer to oval orrectangular in shape in order to provide a clear passageway for thesheet-like electron beam which it is intended to modulate. Obviously,however, this would not be necessary if the present invention wereembodied in a tube using electron beams having narrow cross-sections, i.e., cross-sections not much wider than they are thick.

It is a common practice in the traveling wave tube art to employ strongaxial magnetic fields for the purpose of compelling the beam electronsto move substantially rectilinearly along the drift space despite anyinfluences, such as any initial radial velocity and/or the influence ofthe elec tron space charge of the beam, which tend to cause spreading ofthe beam and impingement of its electrons on the helix. Even though thepresent form of traveling wave tube does not include a helix,nevertheless uncontrolled spreading of any of the beams would be veryundesirable if it could lead to impingement of electrons on inner wall 5of the envelope 3|. Obviously it is not permissible to use any axialmagnetic field for controlling the electron flow as such a field wouldlose its identity by becoming combined with the transverse field, which.is usedfor producing. the: converging: electron trajectories; and; thetwo;

wouldgbe-repla-ced. by a single-resultant-field inappropriate eitherfor. producing: such; trajectories or: for' effecting. rectilinear,flow;

However, thearrangement which has .been: de-.-

scribed herein .will perform;satisfactorilywithoutthe.use..of' an axialmagnetic fie1d.. Ihis is due,

onthe'. one hand, to: the factithat the'xtransverse magnetic field.willv restrict the. electrons: from spreading in directions to; thicken;the; sheet-like forms: of. their-respective beams, i. e:, in movingthroughethe common drift spacethe electrons will 1 tend: to. be confinedwithin. the. trough of; zero: flux, and; onztheotherrhand,.to'thenfactthat. the magnitude: ofi any "spreading of the sheet-shaped; beams inits width. dimension will be relatively quite small. sincethe. rate" atwhich spreading occurs is: very small interms of the=transit:timethrough the; common. drift space: and since, 1111-. like the helix'in'asin'gle-beam growing-wave tube;

the envelope does not have to be fitted closely around: the beam;

WhilerIi have" shown envelopes: each;of which:

compriseszaimaini closure an'd'a numberof branch closures; torprovide: acommon": drift path: anda numberv of' individual drift paths, it would.be: entirely'possible-to form the envelope withonly one large mainclosure which surrounds enough evacuated space toprovide'. room for allof the difierent' drift paths.

Likewise, the envelopes of tubes of: this type" need not necessarily-beformed of metal butamay be of glass. magnetic" conductive coating couldbeapplied' to theenvelope to serve as the: electrostatic shield.

requiredto produce'a field-I-free space inside the envelope.

It is: obvious that. a tube. of. the type. shown: in: Fig-.14 willbe'operative over a narrower band of." frequencies than. that shown in.Fig: 5; For

this reason the embodi'mentiof Fig; 4 might more.

often: be. preferred for use as a part of an. oscillatonxii e., inan'arrangement?" including a feedsback: line: of suitable length connected?between.

the" output and the: input; On: the. other. hand,

the embodiment of Fig. 5 is. operable over an' unusually wide band: of?frequencies so that on this: account;1and due to its great sensitivity"and:

improved signal-tornoise ratio; it? might often be: 7

pref'erredas an amplifier tobe utilized over wide bands at super' highfrequencies.

While" 1'. have indicated the preferred embodiments of'my invention ofwhich=Ii am now. aware and have: also indicatedonly certainspecificapplications:forwhich my invention may be employed", it willibe'apparent that thisinvention-isby no means limited to the exact formsillustratedior the uses indicated; but thatzin practicing it manyvariations may: be made in. the; particular struc'- tures and the.particular uses therefor'which are disclosed herein without departingfrom the scope of'kmy: invention as set forth in the appendedclaims;

is. claimed: is:

LA growing wave' electron-beam: tube comprising. an evacuated envelopeenclosing-a numher. of electron guns in spaced relationship'to'each'other'and defining theboundaries .of'a space affording unobstructedpaths including an elongated substantiallyrectilinear common drift'pathand. a number. of individual". curved drift paths each of'which extendsbetween thebeam output: endofa respective one of said gunsand the-beaminput end of thecommon driftpath, eachof said.

Insuch an embodiment. a none 121 electrongunsbeing elec-trostaticallyshieldedfromthe. others, means providing electrostaticshieldingaroundallof saidpathsg. each; of said; guns. being positioned. so thatits. beam: axis is tangential to-the curved. patlr extending between.itand said common. path, saidenvelopecomprising portions which surroundzall of. saidg-paths andv are made of non-magnetic. material.

2. Aplura-l-beam growing-wave tube comprisingan-envelope including amain. enclosure surrounding a common drift path and aanumberof branchenclosures each surrounding, an individual curved drift: path. which. isconvergent into said common drift path, an electron; gun positioned ineach of the branch enclosures tordirect a'beam of electrons tangentiallytoward apoint on the'curved drift path surroundedby the enclosure, ineach of theIbranch enclosures means for modulating the beam ofelectrons, which traverses its curved path, at a point which islocatedbetween theirespective electron gun for that branch enclosure: and itspoint. of juncture with said main" enclosure;

3.- A plural-beam growing-wave tube as in claim 2- and including aninductive output meansand; the branch enclosure compriseparts of the"evacuated envelope of thetube, each input cavitybeing. positioned inthebranch enclosurebetween said region thereof where an'electron gun isenclosedand'the-point of juncture thereof with the main enclosure, andanoutput resonant cavity connected to the'main enclosure near to itsopposite end byjunctures similar to thoseabove recited with reference-tothe connection of each input cavity into'a' branch enclosure;

5'; A' plural-beam growing-wave tube as in claim 4'whichfurthercomprises-a collectorad-- joined to the end of the main enclosure beyondthe location of the output cavity, the collector being adjoined to saidopposite end by an insulating vacuum-tight seal, and electrostaticshielding"meansbetween theicurved paths of the different beams.

6. A plural-beam growing-wave tube as in claim"5 in which each of saidinput cavities has an individual input coaxial line.

7 A plural-beam growing-wave tube comprising asubstantially rectilinearmain tubularenclosure and two'curved-branch tubular enclosures extendingtherefrom in opposite directions to regions which are-spaced from oneanother, an electrongun'supported in-each of the branch enclosures inone of said regions, each electron gun being positioned with its axistangential to a. centralaxisof its respective branch enclosure.-

8; An electron discharge device of thegrowing-wavetype-including an.elongated envelope, a" plurality 'of cathode means in spacedrelationship to each-other at one end thereof for directing a plurality;of electron beams along predetermined'paths; each'of said'paths beingindividual in a first portion thereof, which is' near to arespectivezcathode, and common along azsecond portion thereofiacollector'at the other end of 1 saidenvelope and of said; commonportions ofv said paths, conductive non-magnetic shielding meanssurrounding all portions of all of said paths, a plurality of signalinput means each mounted across the individual portion of the path of adifferent one of said beams for individually modulating that beam, andmagnetic means mounted externally of said envelope to produce a magneticfield having a flux which is transverse to any of said paths, isnon-uniform in accordance with a predetermined law in the regions alongthe individual first portion of each of the paths and has a trough ofsubstantially zero density along the common second portions of saidpaths.

9. An electron discharge device as in claim 8 and including a signaloutput means located near to said other end of the envelope forextracting amplified signal energy from electron waves moving therein.

10. An electron discharge device comprising a plurality of spacedelectron guns for producing respective electron beams in regionssubstantially separated from each other, output means at a greaterdistance from any of said guns than the distance between any two ofthem, deflection means consisting solely of magnetic means adjacent saidguns for causing each of said beams to follow a curved path which mergeswith the path of each other beam and to extend along a commonrectilinear path, said common path being of substantial length withrespect to the separation between the guns and terminating near saidoutput means.

11. An electron discharge device comprising two spaced-apart electronguns for producing two beams along initially individual respective pathslying in a common plane, magnetic deflection means adjacent the guns forcausing said individual paths to converge into a common rectilinear pathof substantial length with respect to the spacing between said guns,said guns being immersed in the flux provided by said means and saidflux extending at right angles to said plane in the regions of saidindividual and common paths and in the regions Wherein said guns areimmersed in it and output means near the end of said common rectilinearpath.

12. An electron discharge device as in claim 10, further includingnon-magnetic conductive means electrostatically shielding said electronguns from each other and providing electrostatic shielding around all ofsaid paths.

WELLESLEY J. DODDS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,245,627 Varian June 17, 19412,289,319 Strobel July 7, 1942 2,348,133 Iams May 2, 1944 2,406,370Hansen et a1 Aug. 27, 1946 2,407,667 Kircher Sept. 17, 1946 2,457,980 DeForest Jan. 4. 1949

